| General | |
|---|---|
| Symbol | 242Pu |
| Names | plutonium-242 |
| Protons(Z) | 94 |
| Neutrons(N) | 148 |
| Nuclide data | |
| Half-life(t1/2) | 375000 years[1] |
| Isotope mass | 242.059741[2]Da |
| Decay products | 238U |
| Decay modes | |
| Decay mode | Decay energy (MeV) |
| alpha decay | 4.984[3] |
| Isotopes of plutonium Complete table of nuclides | |
Plutonium-242 (242Pu orPu-242) is the second longest-livedisotope of plutonium, with ahalf-life of 375,000 years. The half-life of242Pu is about 15 times that of239Pu; so it is one-fifteenth as radioactive, and not one of the larger contributors tonuclear waste radioactivity.242Pu'sgamma ray emissions are also weaker than those of the other isotopes.[4] As the direct parent ofuranium-238 it is part of theuranium series decay chain.
It is notfissile (but it isfissionable byfast neutrons), and itsneutron capturecross section is low. Like the other even isotopes of plutonium it has a significant rate ofspontaneous fission.
Plutonium-242 is produced by successiveneutron capture on239Pu,240Pu, and241Pu. The odd-mass isotopes239Pu and241Pu have about a 3/4 chance of undergoingfission on capture of athermal neutron and about a 1/4 chance of retaining theneutron and becoming the following isotope. The proportion of242Pu is low at lowburnup but increases faster than linearly due to the intermediate isotopes' buildup.
242Pu has a particularly lowcross section forthermal neutron capture; and it takes three neutron absorptions to become anotherfissile isotope (curium-245) and then one more neutron to undergofission. Even then, there is a chance of the fourth neutron being absorbed instead of fissioning, leading to curium-246 (with again only a small neutron cross-section), so the mean number of neutrons absorbed until fission is even higher than 4. Therefore,242Pu is particularly unsuited to recycling in athermal reactor and would be better used in afast reactor where it can be fissioned directly. However,242Pu's low cross section means that relatively little of it is transmuted during one cycle in a thermal reactor.